Systems and method for exhaust gas recirculation

ABSTRACT

Various systems and methods are provided for exhaust gas recirculation, including an exhaust gas recirculation (EGR) system includes an EGR passage coupling an engine exhaust system to an engine intake system, a first EGR cooler positioned in the EGR passage, the first EGR cooler configured to cool EGR with a first fluid, and a second EGR cooler positioned in the EGR passage downstream of the first EGR cooler, the second EGR cooler configured to cool EGR with a second fluid.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of, and claims priority to, U.S. patentapplication Ser. No. 15/082,106 filed 28 Mar. 2016, which isincorporated by reference herein in its entirety.

BACKGROUND Technical Field

Embodiments of the subject matter disclosed herein relate to enginesystems.

Discussion of Art

In order to meet emissions standards mandated by various emissionsregulating agencies, internal combustion engines may be configured withvarious aftertreatment devices, such as selective catalytic reductionsystems, and/or with exhaust gas recirculation (EGR) to lower emissionproduction and remove emissions from the exhaust. For example, EGR mayreduce peak combustion temperatures, thus lowering NOx emissions. EGRsystems may include an EGR cooler configured to cool the EGR prior tomixing with intake air in order to further reduce combustiontemperatures. The EGR cooler may be a liquid-to-air heat exchanger thatcools the EGR via coolant from an engine coolant system, for example.While such a configuration adequately cools the EGR, the thermalgradient across the EGR cooler may be relatively large due to the highexhaust gas temperature and the lower-temperature coolant at the inletof the EGR cooler. This temperature gradient may lead to EGR coolerperformance issues or EGR cooler degradation.

BRIEF DESCRIPTION

In one embodiment, an exhaust gas recirculation (EGR) system includes anEGR passage coupling an engine exhaust system to an engine intakesystem, a first EGR cooler positioned in the EGR passage, and a secondEGR cooler positioned in the EGR passage downstream of the first EGRcooler. The first EGR cooler is configured to cool EGR with a firstfluid, and the second EGR cooler is configured to cool EGR with a secondfluid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a vehicle system with an EGR system according to a firstembodiment.

FIG. 2 shows a vehicle system with an EGR system according to a secondembodiment.

FIG. 3 shows a vehicle system with an EGR system according to a thirdembodiment.

FIG. 4 is a flow chart illustrating an embodiment of a method forcontrolling an EGR system.

DETAILED DESCRIPTION

The following description relates to embodiments of systems for reducingthe thermal load of an exhaust gas recirculation (EGR) cooler. In oneembodiment, the EGR upstream of the EGR cooler may be “pre-cooled” via asecondary cooler that receives a relatively high temperature coolingfluid (e.g., higher temperature than a temperature of the cooling fluidof the primary EGR cooler), such as exhaust gas from downstream of aturbine. By pre-cooling the EGR via the secondary cooler, the EGRentering the primary EGR cooler may be at a lower temperature, loweringthe thermal gradient at the primary EGR cooler.

In another embodiment, lower-temperature exhaust may be used as EGR. Forexample, exhaust gas from downstream of a turbine may replace or bemixed with exhaust from upstream of the turbine to form the EGR enteringthe EGR cooler. In doing so, the temperature of the EGR may be loweredto a range that does not create a high thermal gradient at the EGRcooler.

The approach described herein may be employed in a variety of enginetypes, and a variety of engine-driven systems. Some of these systems maybe stationary, while others may be on semi-mobile or mobile platforms.Semi-mobile platforms may be relocated between operational periods, suchas mounted on flatbed trailers. Mobile platforms include self-propelledvehicles. Such vehicles can include on-road transportation vehicles, aswell as mining equipment, marine vessels, rail vehicles, and otheroff-highway vehicles (OHV). For clarity of illustration, a locomotive isprovided as an example of a mobile platform supporting a systemincorporating an embodiment of the invention.

Before further discussion of the approach for reducing EGR coolerthermal load, an example of a platform is disclosed in which an enginemay be configured for a vehicle, such as a rail vehicle. For example,FIG. 1 shows a block diagram of an embodiment of a vehicle system 100(e.g., a locomotive system), herein depicted as a rail vehicle 106,configured to run on a rail 102 via a plurality of wheels 110. Asdepicted, the rail vehicle 106 includes an engine 104. In othernon-limiting embodiments, the engine 104 may be a stationary engine,such as in a power-plant application, or an engine in a marine vessel oroff-highway vehicle propulsion system as noted above.

The engine 104 receives intake air for combustion from an intake, suchas an intake manifold 115. The intake may be any suitable conduit orconduits through which gases flow to enter the engine. For example, theintake may include the intake manifold 115, the intake passage 114, andthe like. The intake passage 114 receives ambient air from an air filter(not shown) that filters air from outside of a vehicle in which theengine 104 may be positioned. Exhaust gas resulting from combustion inthe engine 104 is supplied to an exhaust, such as exhaust passage 116.The exhaust may be any suitable conduit through which gases flow fromthe engine. For example, the exhaust may include an exhaust manifold117, the exhaust passage 116, and the like. Exhaust gas flows throughthe exhaust passage 116, and out of an exhaust stack of the rail vehicle106. In one example, the engine 104 is a diesel engine that combusts airand diesel fuel through compression ignition. In other non-limitingembodiments, the engine 104 may combust fuel including gasoline,kerosene, biodiesel, or other petroleum distillates of similar densitythrough compression ignition (and/or spark ignition).

In one embodiment, the rail vehicle 106 is a diesel-electric vehicle. Asdepicted in FIG. 1, the engine 104 is coupled to an electric powergeneration system, which includes an alternator/generator 140 andelectric traction motors 112. For example, the engine 104 is a dieselengine that generates a torque output that is transmitted to thealternator/generator 140 which is mechanically coupled to the engine104. The alternator/generator 140 produces electrical power that may bestored and applied for subsequent propagation to a variety of downstreamelectrical components. As an example, the alternator/generator 140 maybe electrically coupled to a plurality of traction motors 112 and thealternator/generator 140 may provide electrical power to the pluralityof traction motors 112. As depicted, the plurality of traction motors112 are each connected to one of a plurality of wheels 110 to providetractive power to propel the rail vehicle 106. One example configurationincludes one traction motor per wheel. As depicted herein, six pairs oftraction motors correspond to each of six pairs of wheels of the railvehicle. In another example, alternator/generator 140 may be coupled toone or more resistive grids 142. The resistive grids 142 may beconfigured to dissipate excess engine torque via heat produced by thegrids from electricity generated by alternator/generator 140.

In the embodiment depicted in FIG. 1, the engine 104 is a V-12 enginehaving twelve cylinders. In other examples, the engine may be a V-6,V-8, V-10, V-16, I-4, I-6, I-8, opposed 4, or another engine type. Asdepicted, the engine 104 includes a subset of non-donor cylinders 105,which includes six cylinders that supply exhaust gas exclusively to anon-donor cylinder exhaust manifold 117, and a subset of donor cylinders107, which includes six cylinders that supply exhaust gas exclusively toa donor cylinder exhaust manifold 119. In other embodiments, the enginemay include at least one donor cylinder and at least one non-donorcylinder. For example, the engine may have four donor cylinders andeight non-donor cylinders, or three donor cylinders and nine non-donorcylinders. In some examples, the engine may have an equal number ofdonor and non-donor cylinders. In other examples, the engine may havemore donor cylinders than non-donor cylinders. In still furtherexamples, the engine may be comprised entirely of donor cylinders. Itshould be understood, the engine may have any desired numbers of donorcylinders and non-donor cylinders. Further, in some embodiments, thedonor cylinders only supply exhaust gas to the donor cylinder exhaustmanifold and not to the non-donor cylinder exhaust manifold. In someembodiments, the non-donor cylinders only supply exhaust gas to thenon-donor cylinder exhaust manifold and not to the donor cylinderexhaust manifold.

As depicted in FIG. 1, the non-donor cylinders 105 are coupled to theexhaust passage 116 to route exhaust gas from the engine to atmosphere(after it passes through first and second turbochargers 120 and 124, andin some embodiments, through aftertreatment system 130). The donorcylinders 107, which provide engine exhaust gas recirculation (EGR), arecoupled exclusively to an EGR passage 165 of an EGR system 160 whichselectively routes exhaust gas from the donor cylinders 107 to theintake passage 114 of the engine 104 or to atmosphere via the exhaustpassage 116. By introducing cooled exhaust gas to the engine 104, theamount of available oxygen for combustion is decreased, thereby reducingcombustion flame temperatures and reducing the formation of nitrogenoxides (e.g., NO_(x)). Additional details regarding EGR system 160 willbe provided below.

As depicted in FIG. 1, the vehicle system 100 further includes atwo-stage turbocharger with the first turbocharger 120 and the secondturbocharger 124 arranged in series, each of the turbochargers 120 and124 arranged between the intake passage 114 and the exhaust passage 116.The two-stage turbocharger increases air charge of ambient air drawninto the intake passage 114 in order to provide greater charge densityduring combustion to increase power output and/or engine-operatingefficiency. The first turbocharger 120 operates at a relatively lowerpressure, and includes a first turbine 121 which drives a firstcompressor 122. The first turbine 121 and the first compressor 122 aremechanically coupled via a first shaft 123. The first turbocharger maybe referred to the “low-pressure stage” of the turbocharger. The secondturbocharger 124 operates at a relatively higher pressure, and includesa second turbine 125 which drives a second compressor 126. The secondturbocharger may be referred to the “high-pressure stage” of theturbocharger. The second turbine and the second compressor aremechanically coupled via a second shaft 127.

As explained above, the terms “high pressure” and “low pressure” arerelative, meaning that “high” pressure is a pressure higher than a “low”pressure. Conversely, a “low” pressure is a pressure lower than a “high”pressure.

As used herein, “two-stage turbocharger” may generally refer to amulti-stage turbocharger configuration that includes two or moreturbochargers. For example, a two-stage turbocharger may include ahigh-pressure turbocharger and a low-pressure turbocharger arranged inseries, three turbocharger arranged in series, two low pressureturbochargers feeding a high pressure turbocharger, one low pressureturbocharger feeding two high pressure turbochargers, etc. In oneexample, three turbochargers are used in series. In another example,only two turbochargers are used in series.

In the embodiment shown in FIG. 1, the second turbocharger 124 isprovided with a turbine bypass valve 128 which allows exhaust gas tobypass the second turbocharger 124. The turbine bypass valve 128 may beopened, for example, to divert the exhaust gas flow away from the secondturbine 125. In this manner, the rotating speed of the compressor 126,and thus the boost provided by the turbochargers 120, 124 to the engine104 may be regulated during steady state conditions. Additionally, thefirst turbocharger 120 may also be provided with a turbine bypass valve.In other embodiments, only the first turbocharger 120 may be providedwith a turbine bypass valve, or only the second turbocharger 124 may beprovided with a turbine bypass valve. Additionally, the secondturbocharger may be provided with a compressor bypass valve 129, whichallows gas to bypass the second compressor 126 to avoid compressorsurge, for example. In some embodiments, first turbocharger 120 may alsobe provided with a compressor bypass valve, while in other embodiments,only first turbocharger 120 may be provided with a compressor bypassvalve.

While not shown in FIG. 1, in some examples two low-pressureturbochargers may be present. As such, two charge air coolers (e.g.,intercoolers) may be present, one positioned downstream of eachlow-pressure compressor. In one example, the low-pressure turbochargersmay be present in parallel, such that charge air that flows through eachlow-pressure compressor is combined and directed to the high-pressurecompressor.

While in the example vehicle system described herein with respect toFIG. 1 includes a two-stage turbocharger, it is to be understood thatother turbocharger arrangements are possible. In one example, only asingle turbocharger may be present. In such cases, only one charge aircooler may be utilized, rather than the two coolers depicted in FIG. 1(e.g., intercooler 132 and aftercooler 134). In some examples, aturbo-compounding system may be used, where a turbine positioned in theexhaust passage is mechanically coupled to the engine. Herein, energyextracted from the exhaust gas by the turbine is used to rotate thecrankshaft to provide further energy for propelling the vehicle system.Still other turbocharger arrangements are possible.

The vehicle system 100 optionally includes an exhaust treatment system130 coupled in the exhaust passage in order to reduce regulatedemissions. As depicted in FIG. 1, the exhaust gas treatment system 130is disposed downstream of the turbine 121 of the first (low pressure)turbocharger 120. In other embodiments, an exhaust gas treatment systemmay be additionally or alternatively disposed upstream of the firstturbocharger 120. The exhaust gas treatment system 130 may include oneor more components. For example, the exhaust gas treatment system 130may include one or more of a diesel particulate filter (DPF), a dieseloxidation catalyst (DOC), a selective catalytic reduction (SCR)catalyst, a three-way catalyst, a NO_(x) trap, and/or various otheremission control devices or combinations thereof. However, in someexamples the exhaust aftertreatment system 130 may be dispensed with andthe exhaust may flow from the exhaust passage to atmosphere withoutflowing through an aftertreatment device.

Additionally, in some embodiments, the EGR system 160 may include an EGRbypass passage 161 that is coupled to EGR passage 165 and is configuredto divert exhaust from the donor cylinders back to the exhaust passage.The EGR bypass passage 161 may be controlled via a first valve 164. Thefirst valve 164 may be configured with a plurality of restriction pointssuch that a variable amount of exhaust is routed to the exhaust, inorder to provide a variable amount of EGR to the intake.

The flow of EGR to the intake system via EGR passage 165 may becontrolled by a second valve 170. For example, when second valve 170 isopen, exhaust may be routed from the donor cylinders to one or more EGRcoolers (explained in more detail below) and/or additional elementsprior to being routed to the intake passage 114. The first valve 164 andsecond valve 170 may be on/off valves controlled by the control unit 180(for turning the flow of EGR on or off), or they may control a variableamount of EGR, for example. In some examples, the first valve 164 may beactuated such that an EGR amount is reduced (exhaust gas flows from theEGR passage 165 to the exhaust passage 116). In other examples, thefirst valve 164 may be actuated such that the EGR amount is increased(e.g., exhaust gas flows from the donor cylinder manifold to the EGRpassage 165). In some embodiments, the alternate EGR system may includea plurality of EGR valves or other flow control elements to control theamount of EGR.

In such a configuration, the first valve 164 is operable to routeexhaust from the donor cylinders to the exhaust passage 116 of theengine 104 and the second valve 170 is operable to route exhaust fromthe donor cylinders to the intake passage 114 of the engine 104. Assuch, the first valve 164 may be referred to as an EGR bypass valve,while the second valve 170 may be referred to as an EGR metering valve.EGR that flows in EGR passage 165 only flows from the donor cylindersand does not flow from the non-donor cylinders; all exhaust from thenon-donor cylinders flows to atmosphere via exhaust passage 116. In theembodiment shown in FIG. 1, the first valve 164 and the second valve 170may be engine oil, or hydraulically, actuated valves, for example, witha shuttle valve (not shown) to modulate the engine oil. In someexamples, the valves may be actuated such that one of the first andsecond valves 164 and 170 is normally open and the other is normallyclosed. In other examples, the first and second valves 164 and 170 maybe pneumatic valves, electric valves, or another suitable valve.

Exhaust gas flowing from the donor cylinders 107 to the intake passage114 passes through one or more a heat exchangers such as a first EGRcooler 166 and a second EGR cooler 167 to reduce a temperature of (e.g.,cool) the exhaust gas before the exhaust gas returns to the intakepassage. In order to reduce the thermal gradient across the one or moreEGR coolers, the EGR may be “pre-cooled” via the first EGR cooler 166 inorder to lower the temperature of the EGR entering the second EGR cooler167. The temperature of the cooling fluid used to pre-cool the EGR inthe first EGR cooler may be higher than a temperature of the coolingfluid used to cool the EGR in the second EGR cooler. In this way, theoverall temperature reduction of the EGR may be split across two EGRcoolers, lowering the thermal gradient to which each cooler is exposed.

As shown, first EGR cooler 166 is positioned upstream of second EGRcooler 167 in an EGR flow direction. The first EGR cooler 166 isconfigured to cool EGR with a first fluid and the second EGR isconfigured to cool EGR with a second fluid. In some examples, explainedin more detail below, the first fluid may be different than the secondfluid and may be at a different (e.g., higher) temperature than thesecond fluid. However, in other examples, the first and second fluidsmay be the same fluid (e.g., liquid coolant). In such examples, thefirst fluid may be sourced from a different location than the secondfluid (e.g., the first fluid may be sourced from downstream of theengine while the second fluid may be sourced from downstream of acoolant system radiator) such that the fluids are at differenttemperatures.

In one example, the first fluid may be a liquid coolant and the secondfluid may be a gaseous coolant. For example, the first fluid may beliquid coolant that is at a first, higher temperature (e.g., coolantthat has passed through the engine) and the second fluid may be ambientair at a second, lower temperature. The first temperature may be lowerthan EGR temperature upstream of the first EGR cooler 166.

In another example, the first fluid may be gaseous coolant and thesecond fluid may be liquid coolant, such that first EGR cooler 166 is anair-to-air heat exchanger and second EGR cooler is an air-to-liquid heatexchanger. As shown in FIG. 1, the first fluid is exhaust gas fromdownstream of the first turbine 121 and the second fluid is liquidcoolant from a vehicle coolant system 194. However, in other examples,the first fluid may be exhaust gas from downstream of the second turbine125 or from downstream of the aftertreatment system 130, or may beintake or ambient air or other gaseous coolant. Likewise, the secondfluid may be liquid coolant from a source other than the vehicle coolantsystem 194, such as a dedicated coolant system that does not cool othervehicle components. The liquid coolant may comprise water, oil, fuel,refrigerant, or other suitable liquid coolant.

Thus, as shown in FIG. 1, the first EGR cooler 166 is configured to coolEGR with exhaust gas from downstream of the first turbine 121. Thesecond EGR cooler 167 is positioned downstream (in an EGR flowdirection) from the first EGR cooler 166 and is configured to cool EGRwith liquid coolant from coolant system 194. The EGR passage 165 isconfigured to direct EGR from the engine exhaust system upstream of thesecond turbine 125 to the first EGR cooler 166, from the first EGRcooler 166 to the second EGR cooler 167, and from the second EGR cooler167 to the engine intake system (e.g., intake passage 114).

A cooling exhaust flow passage 190 fluidically couples the engineexhaust system downstream of the low-pressure turbine 121 to a coolinginlet of the first EGR cooler 166. An exhaust return passage 191fluidically couples a cooling outlet of the first EGR cooler 166 to theengine exhaust system downstream of the low-pressure turbine 121. Allexhaust gas that flows through the cooling exhaust flow passage 190 alsoflows through the exhaust return passage 191.

The first EGR cooler 166 includes an EGR inlet fluidically coupled tothe EGR passage 165 and an EGR outlet fluidically coupled to the secondEGR cooler 167. The first EGR cooler 166 is configured to flow EGR fromthe EGR inlet to the EGR outlet via one or more EGR flow passages,depicted schematically as the dashed line in first EGR cooler 166. Thefirst EGR cooler 166 is configured to flow exhaust gas from the coolinginlet to the cooling outlet via one or more coolant passages, depictedschematically as the dotted line in first EGR cooler 166. The EGR andexhaust gas are maintained separately throughout an entirety of thefirst EGR cooler 166. As illustrated schematically in FIG. 1, the EGR inthe EGR flow passages does not mix with the exhaust gas in the coolantpassages. Further, the exhaust gas from the beginning of the exhaustpassage 116 (where the exhaust passage 116 couples to the non-donorcylinder manifold) to the end of the exhaust passage 116 (e.g., wherethe exhaust is admitted to atmosphere) does not mix with any EGR fromEGR passage 165. EGR entering the first EGR cooler 166 at the EGR inletis at a higher pressure than exhaust gas entering the first EGR cooler166 at the cooling inlet, at least in some examples. In some examples,the first EGR cooler 166 may be configured (e.g., with a certain heatexchanger area and/or configuration, such as parallel vs. counter-flow)to remove a designated amount of heat from the EGR prior to the EGRentering the second EGR cooler 167, such as cool the EGR by 100 or 200degrees C.

The cooling exhaust flow passage 190 has an inlet coupled to the exhaustpassage 116 of the engine exhaust system downstream of the first turbine121, and the exhaust return passage 191 has an outlet coupled to theexhaust passage 116 downstream of the inlet of the cooling exhaust flowpassage 190. A restriction may be positioned in the exhaust passage 116intermediate the inlet of the cooling exhaust flow passage 190 and theoutlet of the exhaust return passage 191 in order to increase exhaustpressure to allow exhaust gas to flow through cooling exhaust flowpassage 190 and/or reduce exhaust pressure downstream of the restrictionto allow the exhaust gas from the first EGR cooler 166 to be returned tothe exhaust passage 116 via the exhaust return passage 191.

As shown in FIG. 1, the restriction comprises a valve 192. The valve 192may be adjusted via a suitable actuator, such as electric or hydraulicactuator, according to a command sent from the control unit 180, forexample. In some examples, the control unit 180 includes instructions toadjust a position of the valve 192 based on various system operatingparameters, such as one or more of EGR temperature, exhaust gastemperature, a temperature of the liquid coolant, or a temperature ofthe aftertreatment device positioned in the exhaust passage downstreamof the outlet of the exhaust return passage. Additional detailsregarding control of the valve 192 will be presented below with respectto FIG. 4.

A first coolant passage 196 is fluidically coupled to a cooling inlet ofthe second EGR cooler 167 and to coolant system 194. A second coolantpassage 197 is fluidically coupled to a cooling outlet of the second EGRcooler 167 and to coolant system 194. In this way, liquid coolant may besupplied to the second EGR cooler 167 from the coolant system 194 andthen returned to the coolant system 194 after cooling EGR in the secondEGR cooler 167. Coolant system 194 may include one or more suitablecomponents, including but not limited to a radiator, pump, degas bottle,and other coolant lines. The coolant system 194 may also cool the engine104, charge air coolers 132 and 134, and/or other vehicle components.The second EGR cooler 167 includes an EGR inlet fluidically coupled tothe EGR passage 165, downstream of the first EGR cooler 166. The secondEGR cooler 167 also includes an EGR outlet fluidically coupled to an EGRmixer 172 and/or other downstream components, described below. In someexamples, the second EGR cooler 167 may be configured to remove a largeramount of heat from the EGR than the first EGR cooler 166, and as suchmay have a larger heat exchange area.

In some examples, one or more charge air coolers 132 and 134 disposed inthe intake passage 114 (e.g., upstream of where the recirculated exhaustgas enters) may be adjusted to further increase cooling of the chargeair such that a mixture temperature of charge air and exhaust gas ismaintained at a desired temperature. In other examples, the EGR system160 may include one or more EGR cooler bypasses to bypass first EGRcooler 166 and/or second EGR cooler 167. Alternatively, the EGR systemmay include an EGR cooler control element. The EGR cooler controlelement may be actuated such that the flow of exhaust gas through theEGR cooler is reduced; however, in such a configuration, exhaust gasthat does not flow through the EGR cooler may be directed to the exhaustpassage 116 rather than the intake passage 114.

As shown in FIG. 1, the vehicle system 100 further includes an EGR mixer172 which mixes the recirculated exhaust gas with charge air such thatthe exhaust gas may be evenly distributed within the charge air andexhaust gas mixture. In the embodiment depicted in FIG. 1, the EGRsystem 160 is a high-pressure EGR system which routes exhaust gas from alocation upstream of turbochargers 120 and 124 in the exhaust passage116 to a location downstream of turbochargers 120 and 124 in the intakepassage 114. In other embodiments, the vehicle system 100 mayadditionally or alternatively include a low-pressure EGR system whichroutes exhaust gas from downstream of the turbochargers 120 and 124 inthe exhaust passage 116 to a location upstream of the turbochargers 120and 124 in the intake passage 114.

The vehicle system 100 further includes the control unit 180, which isprovided and configured to control various components related to thevehicle system 100. In one example, the control unit 180 includes acomputer control system. The control unit 180 further includesnon-transitory, computer readable storage media (not shown) includingcode for enabling on-board monitoring and control of engine operation.The control unit 180, while overseeing control and management of thevehicle system 100, may be configured to receive signals from a varietyof engine sensors, as further elaborated herein, in order to determineoperating parameters and operating conditions, and correspondinglyadjust various engine actuators to control operation of the vehiclesystem 100. For example, the control unit 180 may receive signals fromvarious engine sensors including sensor 181 arranged in EGR passage 165,sensor 182 arranged in the exhaust passage 116, sensor 183 arranged inthe inlet of the low-pressure compressor, and sensor 184 arranged in theinlet of the high-pressure compressor. The sensors 181, 182, 183, and184 may detect temperature and/or pressure. Sensor 108 positioned in theintake may detect intake oxygen concentration or other suitableparameter. Additional sensors may include, but are not limited to,engine speed, engine load, boost pressure, ambient pressure, enginetemperature, coolant system temperature, etc. Correspondingly, thecontrol unit 180 may control the vehicle system 100 by sending commandsto various components such as traction motors, alternator, cylindervalves, throttle, heat exchangers, valve 192, wastegates or other valvesor flow control elements, EGR valves 164 and/or 170, turbine bypassvalve 128, etc.

Turning to FIG. 2, a second embodiment of a vehicle system 200 isillustrated. Vehicle system 200 includes many similar components tovehicle system 100, and similar components are given like numbers andadditional description is not provided. Vehicle system 200 includes anEGR system 260 that differs from EGR system 160 of FIG. 1 in that EGRsystem 260 includes only one EGR cooler 266. EGR cooler 266 may be aliquid-to-air cooler that receives liquid coolant from a vehicle coolantsystem. EGR from EGR passage 165 flows to EGR cooler 266 and then to EGRmixer 172, as explained above with respect to FIG. 1.

To lower the temperature of the EGR entering EGR cooler 266, the EGRtaken from the donor cylinder manifold may be mixed with exhaust gasfrom downstream of first turbine 121. As such, a low-pressure exhaustpassage 290 may have an inlet coupled to exhaust passage 116 and anoutlet fluidically coupled to EGR passage 165. Valve 292 may bepositioned in low-pressure exhaust passage 290 and be adjustable (via asuitable actuator) to provide a designated amount of low-pressureexhaust to the EGR passage 165, based on EGR demand, EGR temperature,coolant temperature, and/or other parameters.

Because EGR from the donor cylinder manifold is at a higher pressurethan the exhaust gas in low-pressure exhaust passage 290, a mixingdevice 294 may be present at the junction of the low-pressure exhaustpassage 290 and EGR passage 165. The mixing device 294 may include aventuri, check valve, and/or other structure configured to create apressure drop to suction in the low-pressure exhaust and/or prevent thehigher pressure EGR from the donor cylinder manifold from flowing backto the exhaust downstream of the turbine. In one example, thehigh-pressure exhaust from the donor cylinder manifold flowing in theEGR passage may comprise the motive flow of the venturi, while thelow-pressure exhaust gas from the low-pressure exhaust passage may bedrawn in via a suction inlet of the venturi.

As described above with respect to FIG. 1, first valve 164 controls flowof exhaust gas from the donor cylinder manifold to the exhaust passageand second valve 170 controls flow of exhaust gas from the donorcylinder manifold to the EGR passage and subsequently the intakemanifold. EGR that flows in EGR passage 165 upstream of the mixingdevice 294/junction with low-pressure exhaust passage 290 only flowsfrom the donor cylinders and does not flow from the non-donor cylinders;all exhaust from the non-donor cylinders flows to exhaust passage 116.As described previously, first valve 164 and second valve 170 maycontrolled in tandem based on EGR demand (as determined based on atarget intake oxygen fraction, for example).

Additionally, in some examples, first valve 164 and second valve 170 maybe controlled in the same regardless of the position of valve 292. Forexample, if engine EGR demand dictates that first valve 164 and secondvalve 170 each be 50% open, the respective valves will be maintained at50% open even when valve 292 is at least partially open and low-pressureexhaust gas from downstream of the low-pressure turbine is provided toEGR passage 165. However, in other examples, the control of at leastfirst valve 164 may change based on the position of valve 292. Using theexample valve positions described above, first valve 164 and secondvalve 170 may be commanded to be 50% open based on EGR demand when valve292 is fully closed, but if valve 292 opens, first valve 164 may beadjusted to accommodate the increased amount of donor cylinder exhaustthat may flow to the exhaust passage via bypass passage 161 (e.g., theamount of donor cylinder exhaust that is displaced by the low-pressureexhaust gas).

FIG. 3 illustrates a third embodiment of a vehicle system 300. Vehiclesystem 300 includes many similar components to vehicle system 100, andsimilar components are given like numbers and additional description isnot provided. Vehicle system 300 includes an EGR system 360 that differsfrom EGR system 160 of FIG. 1 in that EGR system 360 includes only oneEGR cooler 366. EGR cooler 366 may be a liquid-to-air cooler thatreceives liquid coolant from a vehicle coolant system. EGR fromlow-pressure exhaust passage 390 flows to EGR cooler 366 and then to EGRmixer 172 and eventually the intake passage 114.

To lower the temperature of the EGR entering EGR cooler 366, the exhaustgas from the donor cylinder manifold 119 may be routed to the exhaustpassage 116 via passage 365, and the exhaust sourced for the EGR istaken from downstream of the first turbine 121. As such, thelow-pressure exhaust passage 390 may have an inlet coupled to exhaustpassage 116 and an outlet fluidically coupled to EGR cooler 366. Valve392 may be positioned in low-pressure exhaust passage 390 and beadjustable (via a suitable actuator) to provide a designated amount oflow-pressure exhaust to the EGR cooler 366, based on output from sensor381 (which may sense low-pressure exhaust flow rate, pressure, oxygencontent, and/or other suitable parameter) and a designated intake oxygenamount, for example. A device 394 may be positioned downstream of EGRcooler 366 and upstream of mixer 172.

The device 394 may include a ventui to draw the low-pressure exhaustand/or a check valve to prevent entry of high-pressure charge air fromdownstream of cooler 134 into passage 390.

Because EGR is sourced from downstream of the first turbine 121 and notfrom upstream of the second turbine 125, the high-pressure donorcylinder EGR is dispensed with, and all exhaust gas from both the donorcylinder manifold and non-donor cylinder manifold is mixed in theexhaust passage 116.

FIG. 4 is a flow chart illustrating a method 400 for controlling an EGRsystem, such as the EGR system 160 of FIG. 1, EGR system 260 of FIG. 2,or EGR system 360 of FIG. 3. Method 400 may be carried out by a controlunit, such as control unit 180, according to non-transitory instructionsstored in memory of the control unit.

At 402, method 400 includes determining operating parameters. Thedetermined operating parameters may include engine speed, engine load,engine temperature, exhaust gas temperature (as sensed by sensor 182,for example), EGR temperature (as sensed by sensor 181, for example),coolant system coolant temperature, and other parameters. At 404, method400 includes adjusting one or more EGR valves to deliver a designatedEGR fraction to an intake of an engine. The one or more EGR valves mayinclude an EGR bypass valve and/or EGR metering valve, such as firstvalve 164 and second valve 170 of FIG. 1, or may include a low-pressureexhaust valve, such as valve 292 of FIG. 2 or valve 392 of FIG. 3. TheEGR valve(s) may be adjusted to provide an EGR amount (e.g., intakefraction, flow rate, or other suitable amount) based on sensed intakeoxygen fraction (from sensor 108, for example) and a target intakeoxygen concentration, for example, as indicated at 403. In otherexamples, the EGR valve(s) may be adjusted based on engine speed, engineload, notch throttle position, or other parameters.

Further, as explained above, one example EGR system (e.g., EGR system260 of FIG. 2) may mix EGR from the donor manifold with exhaust gas fromdownstream of the turbine to reduce the temperature of the EGR enteringthe EGR cooler. The amount of exhaust gas directed from downstream ofthe turbine may be based on the EGR demand, as explained above, andfurther based on the temperature of the EGR (as sensed by sensor 181,for example), the temperature of the exhaust gas downstream of theturbine, and/or the temperature of the coolant in the EGR cooler. Thus,as indicated at 405, the EGR valve(s) may be further adjusted based onEGR, exhaust gas, and/or coolant temperature.

At 406, method 400 includes flowing EGR through one or more of a firstEGR cooler or a second EGR cooler, such as first EGR cooler 166 andsecond EGR cooler 167 of FIG. 1, or EGR cooler 266 or 366. After flowingthrough the EGR cooler(s), the EGR is directed to the intake of theengine. As explained above with respect to FIG. 1, in some examples, theEGR may first flow through the first EGR cooler and may then flowthrough the second EGR cooler.

At 408, method 400 optionally includes adjusting an amount of coolingexhaust directed to the first EGR cooler. As explained above, the firstEGR cooler may be a secondary cooler that pre-cools the EGR usingexhaust gas from downstream of a turbine, such as turbine 121 of FIG. 1.The amount of exhaust from downstream of the turbine that flows to thefirst EGR cooler may be controlled via a valve, such as valve 192 ofFIG. 1. The valve may be adjusted to create suitable exhaust pressureconditions that allow for exhaust from downstream of the turbine to bedirected to the EGR cooler and then back to the exhaust passage, asexplained above with respect to FIG. 1. Further, the valve may beadjusted based on the amount of EGR flowing through the first EGRcooler. For example, the valve may be fully opened when EGR is disabled,and thus little or no cooling exhaust gas may be routed to the first EGRcooler. When the valve is at a more closed or fully closed position,pumping losses and/or turbine outlet pressure may be increased, reducingengine efficiency, and thus when there is no cooling demand at the firstEGR cooler, it may be advantageous to prevent the cooling exhaust gasfrom flowing to the first EGR cooler.

In some examples, the valve may be additionally or alternativelyadjusted based on various system temperatures. In one example, asindicated at 410, the valve may be adjusted based on EGR and coolingexhaust temperatures. For example, if the cooling exhaust (exhaust fromdownstream of the turbine) is at a relatively high temperature (e.g.,within a threshold range of the EGR temperature, such as within 100degrees C.), the valve may be adjusted to a more open position to directless cooling exhaust to the first EGR cooler. If the cooling exhaust isat a relatively low temperature (e.g., 100 degrees C. or more lower thanthe EGR temperature), the valve may be adjusted to a more closedposition to flow more cooling exhaust to the first EGR cooler. In thisway, when the cooling exhaust is the same temperature (or notsignificantly cooler) than the EGR temperature, the cooling exhaust maybe directed to atmosphere without passing through the first EGR cooler,reducing the pumping losses and/or increased turbine outlet pressurethat may be associated with flowing the cooling exhaust to the first EGRcooler.

In some examples, the valve may be adjusted based on liquid coolanttemperature, as indicated at 412. For example, if the liquid coolanttemperature is equal to or higher than the cooling exhaust temperature,no additional cooling benefits may be gained by flowing the coolingexhaust to the first EGR cooler, and thus the valve may be moved to amore open position.

Further, the valve may be adjusted based on an aftertreatment devicetemperature, as indicated at 414. The aftertreatment device, such asaftertreatment system 130 of FIG. 1, may include a catalyst thatperforms optimally when at a certain temperature, referred to as thelight-off temperature. If the aftertreatment device is below thelight-off temperature, diverting exhaust to the first EGR cooler maydelay aftertreatment device warm-up and thus the valve may be moved to amore open (e.g., fully open) position to direct as much exhaust gas aspossible to the aftertreatment device. In contrast, if theaftertreatment device temperature is above an upper thresholdtemperature, the aftertreatment device may degrade and thus the valvemay be moved to a more closed position (e.g., fully closed) to route asmuch exhaust gas as possible to the first EGR cooler. In doing so, theexhaust gas that reaches the aftertreatment device may be lower intemperature than if the exhaust gas was directly routed to theaftertreatment device. Such a control may be independent of coolingdemands at the first EGR cooler (e.g., when aftertreatment devicetemperature is high, the valve may be closed even when the exhaust gasis not needed for cooling, such as when EGR temperature is lower thanexhaust gas temperature). Method 400 then returns.

In this way, the thermal load on a given EGR cooler may be reduced bysplitting the cooling of the EGR between two separate EGR coolers. Inother examples, the temperature of the EGR may be reduced prior toentering the EGR cooler by mixing the EGR with exhaust gas fromdownstream of a turbine, or by fully replacing the high-pressure EGRwith low-pressure EGR. By doing so, the thermal gradient at an EGRcooler may be reduced, lowering stress placed on the EGR cooler andprolonging the life of the EGR cooler.

An embodiment relates to an exhaust gas recirculation (EGR) system,including an EGR passage coupling an engine exhaust system to an engineintake system, a first EGR cooler positioned in the EGR passage, thefirst EGR cooler configured to cool EGR with a first fluid, and a secondEGR cooler positioned in the EGR passage downstream of the first EGRcooler, the second EGR cooler configured to cool EGR with a secondfluid.

In one example, the first fluid comprises liquid coolant and the secondfluid comprises exhaust gas from downstream of a turbine. In anotherexample, the first fluid comprises exhaust gas from downstream of aturbine and the second fluid comprises liquid coolant. In such anexample, the EGR passage may be configured to direct EGR from the engineexhaust system upstream of the turbine to the first EGR cooler, from thefirst EGR cooler to the second EGR cooler, and from the second EGRcooler to the engine intake system. Such an example may further comprisea cooling exhaust flow passage fluidically coupling the engine exhaustsystem downstream of the turbine to a cooling inlet of the first EGRcooler. Such an example may further comprise an exhaust return passagefluidically coupling a cooling outlet of the first EGR cooler to theengine exhaust system downstream of the turbine, wherein all exhaust gasthat flows through the cooling exhaust flow passage also flows throughthe exhaust return passage.

Such an example may include the first EGR cooler comprising an EGR inletfluidically coupled to the EGR passage and an EGR outlet fluidicallycoupled to the second EGR cooler. Such an example may include the firstEGR cooler being configured to flow EGR from the EGR inlet to the EGRoutlet via one or more EGR flow passages, the first EGR cooler beingconfigured to flow exhaust gas from the cooling inlet to the coolingoutlet via one or more coolant passages, and the EGR and exhaust gasbeing maintained separately throughout an entirety of the first EGRcooler. Such an example may include EGR entering the first EGR cooler atthe EGR inlet being at a higher pressure than exhaust gas entering thefirst EGR cooler at the cooling inlet.

Such an example may include the cooling exhaust flow passage having aninlet coupled to an exhaust passage of the engine exhaust systemdownstream of the turbine, the exhaust return passage having an outletcoupled to the exhaust passage downstream of the inlet of the coolingexhaust flow passage, and further including a restriction in the exhaustpassage intermediate the inlet of the cooling exhaust flow passage andthe outlet of the exhaust return passage. In an example, the restrictioncomprises a valve, and the system may further comprise a controllerconfigured to adjust a position of the valve based on one or more of EGRtemperature, exhaust gas temperature, a temperature of the liquidcoolant, or a temperature of a catalyst positioned in the exhaustpassage downstream of the outlet of the exhaust return passage. Such anexample may further comprise a first coolant passage fluidically coupledto a cooling inlet of the second EGR cooler and a second coolant passagefluidically coupled to a cooling outlet of the second EGR cooler, andthe second EGR cooler may include an EGR inlet fluidically coupled tothe EGR passage.

Another embodiment of a system includes an engine having a first subsetof cylinders and a second subset of cylinders; a first exhaust manifoldcoupled to the first subset of cylinders and a second exhaust manifoldcoupled to the second subset of cylinders; a high-pressure turbochargerturbine coupled in an exhaust passage downstream of the engine; alow-pressure turbocharger turbine coupled in the exhaust passagedownstream of the high-pressure turbocharger turbine; an EGR passagecoupling the first exhaust manifold to an intake manifold of the engine;a first EGR cooler positioned in the EGR passage, the first EGR coolercomprising an air-to-air heat exchanger including an EGR inletfluidically coupled to the EGR passage and a cooling inlet fluidicallycoupled to the exhaust passage downstream of the low-pressureturbocharger turbine; and a second EGR cooler positioned in the EGRpassage downstream of the first EGR cooler, the second EGR coolercomprising a liquid-to-air heat exchanger including an EGR inlet coupledto the EGR passage and a cooling inlet fluidically coupled to a coolantpassage. In an example, the system further includes an exhaust flowcontrol valve positioned in the exhaust passage downstream of thelow-pressure turbocharger turbine, a first EGR valve positioned in theEGR passage upstream of the first EGR cooler, and a second EGR valvepositioned between the first exhaust manifold and the exhaust passage.

Another embodiment of a system includes an engine having a first subsetof cylinders and a second subset of cylinders; a first exhaust manifoldcoupled to the first subset of cylinders and a second exhaust manifoldcoupled to the second subset of cylinders; a high-pressure turbochargerturbine coupled in an exhaust passage downstream of the engine; alow-pressure turbocharger turbine coupled in the exhaust passagedownstream of the high-pressure turbocharger turbine; an EGR passagecoupling the first exhaust manifold to an intake manifold of the engine;and a low-pressure exhaust passage coupling the exhaust passagedownstream of the low-pressure turbine to the EGR passage via a venturi.In an example, the system further comprises a valve positioned in thelow-pressure exhaust passage and a controller configured to adjust aposition of the valve based on one or more of EGR demand or atemperature of EGR in the EGR passage.

An embodiment of a method includes directing exhaust gas from upstreamof a turbocharger turbine to an exhaust gas inlet of a first exhaust gasrecirculation (EGR) cooler and directing exhaust gas from downstream ofthe turbocharger turbine to one or more coolant passages of the firstEGR cooler; and directing exhaust gas from an exhaust gas outlet of thefirst EGR cooler to an exhaust gas inlet of a second EGR cooler, anddirecting cooling system coolant to one or more coolant passages of thesecond EGR cooler.

In one example, directing exhaust gas from downstream of theturbocharger turbine to one or more coolant passages of the first EGRcooler may include adjusting an amount of exhaust gas directed fromdownstream of the turbocharger turbine to one or more coolant passagesof the first EGR cooler based on a temperature of the exhaust gasdownstream of the turbocharger turbine relative to a temperature ofexhaust gas upstream of the turbocharger turbine. In an example,directing exhaust gas from downstream of the turbocharger turbine to oneor more coolant passages of the first EGR cooler may include adjustingan amount of exhaust gas directed from downstream of the turbochargerturbine to one or more coolant passages of the first EGR cooler based ona temperature of a catalyst positioned downstream of the turbochargerturbine. In an example, adjusting the amount of exhaust gas directedfrom downstream of the turbocharger turbine to one or more coolantpassages of the first EGR cooler based on the temperature of thecatalyst may include decreasing the amount of exhaust gas directed fromdownstream of the turbocharger turbine to one or more coolant passagesof the first EGR cooler responsive to catalyst temperature being below afirst threshold temperature. In an example, adjusting the amount ofexhaust gas directed from downstream of the turbocharger turbine to oneor more coolant passages of the first EGR cooler based on thetemperature of the catalyst may include increasing the amount of exhaustgas directed from downstream of the turbocharger turbine to one or morecoolant passages of the first EGR cooler responsive to catalysttemperature being above a second threshold temperature. In an example,directing exhaust gas from downstream of the turbocharger turbine to oneor more coolant passages of the first EGR cooler may include adjustingan amount of exhaust gas directed from downstream of the turbochargerturbine to one or more coolant passages of the first EGR cooler based ona temperature of the cooling system coolant.

As used herein, an element or step recited in the singular and proceededwith the word “a” or “an” should be understood as not excluding pluralof said elements or steps, unless such exclusion is explicitly stated.Furthermore, references to “one embodiment” of the invention do notexclude the existence of additional embodiments that also incorporatethe recited features. Moreover, unless explicitly stated to thecontrary, embodiments “comprising,” “including,” or “having” an elementor a plurality of elements having a particular property may includeadditional such elements not having that property. The terms “including”and “in which” are used as the plain-language equivalents of therespective terms “comprising” and “wherein.” Moreover, the terms“first,” “second,” and “third,” etc. are used merely as labels, and arenot intended to impose numerical requirements or a particular positionalorder on their objects.

This written description uses examples to disclose the invention,including the best mode, and also to enable a person of ordinary skillin the relevant art to practice the invention, including making andusing any devices or systems and performing any incorporated methods.The patentable scope of the invention is defined by the claims, and mayinclude other examples that occur to those of ordinary skill in the art.Such other examples are intended to be within the scope of the claims ifthey have structural elements that do not differ from the literallanguage of the claims, or if they include equivalent structuralelements with insubstantial differences from the literal languages ofthe claims.

What is claimed is:
 1. A system, comprising: an engine having a firstsubset of cylinders and a second subset of cylinders; a first exhaustmanifold coupled to the first subset of cylinders and a second exhaustmanifold coupled to the second subset of cylinders; a high-pressureturbocharger turbine coupled in an exhaust passage downstream of theengine; a low-pressure turbocharger turbine coupled in the exhaustpassage downstream of the high-pressure turbocharger turbine; an exhaustgas recirculation (EGR) passage coupling the first exhaust manifold toan intake manifold of the engine; and a low-pressure exhaust passagecoupling the exhaust passage downstream of the low-pressure turbochargerturbine to the EGR passage via a venturi.
 2. The system of claim 1,further comprising a valve positioned in the low-pressure exhaustpassage and a controller configured to adjust a position of the valvebased on at least one of an EGR demand of the engine and a temperatureof EGR in the EGR passage detected via a sensor.
 3. The system of claim2, further comprising a first valve controlling flow of exhaust gas fromthe first exhaust manifold to the exhaust passage downstream of theengine, and a second valve controlling flow of the exhaust gas from thefirst exhaust manifold to the EGR passage.
 4. The system of claim 3,wherein the first valve and the second valve are controlled in tandembased on the EGR demand of the engine.
 5. The system of claim 3, whereinthe first valve and the second valve are controlled independent of theposition of the valve positioned in the low-pressure exhaust passage. 6.The system of claim 1, wherein high-pressure exhaust from the firstexhaust manifold flowing in the EGR passage comprises a motive flow ofthe venturi, and wherein low-pressure exhaust gas from the low-pressureexhaust passage is drawn in via a suction inlet of the venturi.
 7. Thesystem of claim 1, wherein EGR that flows in the EGR passage upstream ofthe venturi only flows from the first subset of cylinders and does notflow from the second subset of cylinders.
 8. A system, comprising: anengine having a first subset of cylinders and a second subset ofcylinders; a first exhaust manifold coupled to the first subset ofcylinders and a second exhaust manifold coupled to the second subset ofcylinders; a high-pressure turbocharger coupled in an exhaust passagedownstream of the engine, the high-pressure turbocharger comprising ahigh-pressure compressor and a high-pressure turbine, wherein thehigh-pressure turbine is coupled to the second exhaust manifold by theexhaust passage, and wherein exhaust produced by the second subset ofcylinders is routed to the high-pressure turbine by the exhaust passage;a low-pressure turbocharger coupled in the exhaust passage downstream ofthe high-pressure turbocharger, the low-pressure turbocharger comprisinga low-pressure compressor and a low-pressure turbine; an exhaust gasrecirculation (EGR) passage coupling the first exhaust manifold to anintake manifold of the engine; and a low-pressure exhaust passagecoupling the exhaust passage downstream of the low-pressure turbine tothe EGR passage via a venturi.
 9. The system of claim 8, furthercomprising a valve positioned in the low-pressure exhaust passage and acontroller configured to adjust a position of the valve based on atleast one of an EGR demand of the engine and a temperature of EGR in theEGR passage detected via a sensor.
 10. A vehicle comprising: an enginehaving a first subset of cylinders and a second subset of cylinders; analternator configured to be driven by the engine to produce electricalpower; one or more traction motors configured to receive the electricalpower from the alternator to propel the vehicle; a first exhaustmanifold coupled to the first subset of cylinders and a second exhaustmanifold coupled to the second subset of cylinders; a high-pressureturbocharger coupled in an exhaust passage downstream of the engine, thehigh-pressure turbocharger comprising a high-pressure compressor and ahigh-pressure turbine, wherein the high-pressure turbine is coupled tothe second exhaust manifold by the exhaust passage, and wherein exhaustproduced by the second subset of cylinders is routed to thehigh-pressure turbine by the exhaust passage; a low-pressureturbocharger coupled in the exhaust passage downstream of thehigh-pressure turbocharger, the low-pressure turbocharger comprising alow-pressure compressor and a low-pressure turbine; an exhaust gasrecirculation (EGR) passage coupling the first exhaust manifold to anintake manifold of the engine; and a low-pressure exhaust passagecoupling the exhaust passage downstream of the low-pressure turbine tothe EGR passage via a venturi.